What Berthollet Wrote

In response to:

To confuse, as M.F. Perutz does [NYR, June 20, p. 31], the French chemist Claude-Louis Berthollet (1748-1822) with his younger compatriot Marcelin Berthelot (1827-1907), also a chemist, is understandable, and to get the date wrong by a year (1785, not 1784) is not serious. But to claim that the relevant chemist (Berthollet, not Berthelot) “discovered that ammonia consists of one atom of nitrogen and three atoms of hydrogen” perpetrates a few anachronisms worth correcting. What Berthollet wrote in the conclusion to his paper “Analyse de l’Alkali Volatil”* was that:

I sparked 4 measures of [volatile alkali] with a superabundance of vital air in a volta eudiometer. The result of this experiment is that this gas—volatile alkali—contains 2.9 of the inflammable gas from water and 1.1 of mephitic gas…

Your readers will probably be willing to forgo the historical and chemical details. The point is that one cannot readily equate “volatile alkali,” “inflammable gas,” and “mephitic gas” respectively with “ammonia,” “hydrogen,” and “nitrogen,” for the chemical integrity of these substances was still under discussion. But worse: the quantities are given as combining proportions first by volume, then by weight. This way of stating the quantitative relations of composition does not entail the atomic theory, as the long controversy over the worth of that theory through the nineteenth century, attests. Berthollet himself did not accept John Dalton’s atomic theory of chemical combination. I know that Professor Perutz had to be brief. I think, though, that his laconic statement is short to the point of falsification.